Python: Reorganize A2A samples and use package A2AExecutor (#6165)

* Reorganize A2A samples: client demos in 02-agents, use package A2AExecutor

- Move client samples (agent_with_a2a, a2a_agent_as_function_tools) to samples/02-agents/a2a/
- Add new concept samples: polling, stream reconnection, protocol selection
- Replace sample agent_executor.py with package-level A2AExecutor (stream=True)
- Update 04-hosting/a2a to focus on server-side, point to 02-agents for clients
- Add README.md for the new 02-agents/a2a/ sample collection

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix streaming artifact coalescing and address PR review feedback

A2AExecutor fix:
- Generate a stable artifact_id per stream in _run_stream so all streaming
  chunks share the same ID, enabling proper append=True coalescing per the
  A2A spec (TaskArtifactUpdateEvent with same artifactId).
- Previously, item.message_id was None for OpenAI/Foundry streaming updates,
  causing the SDK to generate a new random UUID per token (100+ separate
  artifacts instead of 1 appended artifact).

Sample improvements:
- Replace join workaround with response.text now that coalescing works
- Add background=True to stream reconnection resume call (required for
  continuation token emission on in-progress tasks)
- Fix type ignore specificity in polling sample

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/samples/02-agents/a2a/a2a_protocol_selection.py

@@ -0,0 +1,84 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+import asyncio
+import os
+
+import httpx
+from a2a.client import A2ACardResolver
+from agent_framework.a2a import A2AAgent
+from dotenv import load_dotenv
+
+load_dotenv()
+
+"""
+A2A Protocol Selection
+
+This sample demonstrates how to configure which protocol binding the A2A client
+uses when connecting to a remote agent. The A2A specification defines three
+standard bindings: JSONRPC, GRPC, and HTTP+JSON. Agents declare their supported
+bindings in their AgentCard, and clients can express a preference.
+
+Key concepts demonstrated:
+- Configuring `supported_protocol_bindings` on A2AAgent
+- The client selects a binding that matches the remote agent's capabilities
+- Fallback behavior when preferred binding is unavailable
+
+This is the A2A equivalent of the .NET A2AAgent_ProtocolSelection sample.
+
+Prerequisites:
+- Set A2A_AGENT_HOST to the URL of a running A2A server
+
+To run this sample:
+    cd python/samples/02-agents/a2a
+    uv run python a2a_protocol_selection.py
+"""
+
+
+async def main() -> None:
+    """Demonstrates configuring A2A protocol binding preferences."""
+    a2a_agent_host = os.getenv("A2A_AGENT_HOST")
+    if not a2a_agent_host:
+        raise ValueError("A2A_AGENT_HOST environment variable is not set")
+
+    # 1. Resolve agent card to see what bindings are available.
+    async with httpx.AsyncClient(timeout=60.0) as http_client:
+        resolver = A2ACardResolver(httpx_client=http_client, base_url=a2a_agent_host)
+        agent_card = await resolver.get_agent_card()
+
+    print(f"Agent: {agent_card.name}")
+    print("Supported interfaces:")
+    for interface in agent_card.supported_interfaces:
+        print(f"  - {interface.protocol_binding} @ {interface.url}")
+
+    # 2. Create agent with explicit protocol binding preference.
+    #    The list is ordered by preference — the SDK will select the first
+    #    binding that matches a supported interface on the agent card.
+    #
+    #    This matters when a server exposes multiple interfaces (e.g. JSONRPC
+    #    on / and HTTP+JSON on /api/). If only one binding is available, the
+    #    client uses it regardless of your preference list.
+    async with A2AAgent(
+        name=agent_card.name,
+        agent_card=agent_card,
+        url=a2a_agent_host,
+        supported_protocol_bindings=["HTTP+JSON", "JSONRPC"],
+    ) as agent:
+        print("\nConfigured bindings: ['HTTP+JSON', 'JSONRPC']")
+        response = await agent.run("Tell me a short joke")
+        print(f"Response: {response.text}")
+
+
+if __name__ == "__main__":
+    asyncio.run(main())
+
+
+"""
+Sample output:
+
+Agent: PolicyAgent
+Supported interfaces:
+  - JSONRPC @ http://localhost:5001/
+
+Configured bindings: ['HTTP+JSON', 'JSONRPC']
+Response: Here's a short joke for you...
+"""